In the field of well logging with a tool that is suspended from a cable, accurate information of the depth of the tool is difficult to obtain because the tool tends to bounce up and down as the cable is drawn up the borehole. This is an effect often referred to as "yo-yo" and renders depth correlation of high resolution information inaccurate. Such problem is particularly severe in pad-type tools and deviated wells where the friction sometimes is sufficiently great to stop the tool completely which then races ahead as tension built up in the cable overcomes the static friction. The sticking and slipping and the yo-yo motion of the tool, if not compensated for, can lead to erroneous and misleading results on logs produced by high resolution earth formation investigations from inside a borehole.
For example, in a high resolution dipmeter took with which the angles of beddings are to be determined, errors in the tool speed, or more properly depth, translate directly into errors in computed dips and at times are severe enough to prevent a dip determination altogether. This problem has been well recognized as, for example, described in an article entitled "The High Resolution Dipmeter Tool" by L. A. Allaud and J. Ringot and published in "The Log Analyst" of May-June 1969. The solution proposed in this article involves a pair of electrodes that are vertically spaced from each other by a known distance and assuming these electrodes pass over the same track on the borehole wall, the responses of the electrodes should yield the same current response except for a displacement proportional to the spacing between the electrodes. Tool velocity can be estimated by cross-correlating segments of current measurements from the two electrodes. This technique works well when the tool rotates but slowly with good pad contact with the borehole wall and provided that speed variations about the average are not severe. However, when speed variations are large, such as when the tool motion is jerky (stick and slip), or when the measurement curves from the two electrodes fail to resemble each other sufficiently to correlate well, then this technique does not work well.
Although compensation for tool yo-yo has often been referred to as a speed correction problem, in practice it is a correction of the tool depth measurement made by monitoring the motion of the cable from which the tool is suspended. An improvement in a determination of depth has been obtained by use of accelerometers used to both measure tool acceleration and the tool's inclination relative to the local vertical. This technique employs accelerometers and causes inaccuracies in the depth determination attributable to an assumption that the cable measured depth represents true tool depth at regular depth intervals. This assumption, however, is not consistently correct, particularly because it fails to take tool jerking motions into account at boundaries of such intervals. Furthermore, although in theory accelerometers can yield an accurate determination of depth by use of well known integration techniques, errors in the accelerometer measurements eventually accumulate to significant values, thus requiring periodic updating or correction with other measurements.